The present invention generally relates to power generation, and more particularly, to a method and apparatus for reducing carbon dioxide (CO2) when generating power from carbon-based fuels.
Most power generation is provided by heating a working fluid, such as water to produce steam, and using the heated fluid to turn a turbine connected to a generator, which produces electricity. A boiler is typically used to heat the working fluid. Conventional boilers burn fuel such as coal, producing a flame that heats working fluid flowing through tubes extending through the boiler. The heat energy transferred to the working fluid turns the turbine, which turns the generator connected to the turbine. The working fluid cools as it passes through the turbine. Frequently, the working fluid is directed back through the tubes in the boiler where it is reheated and used again to turn the turbine. Because carbon-based fuels are inexpensive relative to other heat sources, they are frequently used in the boiler. One drawback of using carbon-based fuels is they produce harmful emissions such as CO2 when they burn.
One way of reducing CO2 emissions from carbon-based power generation is oxy-combustion, in which fuel is burning in an environment of oxygen and recycled flue gas to produce high purity CO2. In the past, oxy-combustion has been performed at atmospheric pressure and recycled flue gas was regulated to control flame temperature and heat transfer rate in the boiler yielding conditions similar to conventional air-fired boilers. Conventional oxy-combustion is costly and inefficient due to a need to produce oxygen for the process, as well as, a need to compress and purify emitted CO2 before use or disposal.
To reduce cost and increase efficiency associated with capturing and pressurizing CO2, some have suggested burning the fuel in a pressurized atmosphere. Prior pressurized oxy-combustion systems have heated the working fluid with flue gas using a convective heat exchanger (e.g., heat recover steam generator). The tubes in the heat exchanger carrying the working fluid are separated from the burning fuel so the working fluid is not radiantly heated by the flame. In these systems, flue gas is recycled to cool the flue gas entering the heat exchanger to prevent damage to the tubes due to excess gas temperatures. Recycling flue gas results in losses in power plant efficiency. Prior systems required recycled flue gas or inert gas in practice to prevent the heat exchanger from exceeding safe operating temperatures. Accordingly, there continues to be a need for an oxy-combustion system having increased efficiencies and lower operating costs.